twosamples: Fast Permutation Based Two Sample Tests

Fast randomization based two sample tests.
Testing the hypothesis that two samples come from the same distribution
using randomization to create p-values. Included tests are: Kolmogorov-Smirnov,
Kuiper, Cramer-von Mises, and Anderson-Darling. There is also a very efficient test
based on the Wasserstein Distance. The default test 'two_sample' builds on the
Wasserstein distance by using a weighting scheme like that of Anderson-Darling.
We also include the permutation scheme to make test building simple for others.